Generator & Inverter Size Calculator
Running kW, worst motor start, set kVA at its rated power factor and the UPS VA.
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- Free preview: each check’s title, a watermarked starting chart and the names of your first loads (up to 3), with the rating, the working and every figure hidden.
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Generator & Inverter Size Calculator
Size a generating set, an inverter or a UPS from the loads it must carry. List each load as an appliance with its running and starting (surge) watts, or as a motor with its output, efficiency, power factor and locked-rotor data: the NEMA code letter, the locked-rotor amps on the nameplate, or the starting current as a multiple of the rated current. Choose how each motor starts (direct on line, star–delta, autotransformer, soft starter or drive).
The calculator adds the running load as real and reactive power, finds the worst moment, when the motor with the biggest surge starts while everything else runs, and shows which start sets the size. It gives the minimum set rating in kVA and kW at the set’s rated power factor with your margin, the next rating from a list you can edit, the motor-starting kVA the set must supply, the running and surge watts for a portable or inverter generator, and the VA, W and surge an inverter or UPS needs. It also compares starting the motors one at a time with starting them all at once.
Without a pass, the free preview works on your own load list and shows each check’s title, the starting chart (watermarked, without its figures) and your first loads by name; the rating, the starting kVA, every other figure, the working, Copy and the CSV and text reports come with a Pro pass.
How to use it
- Add every load that can run at the same time: Add an appliance for loads given in watts, Add a motor for pumps, compressors and other motors, or pick a common load (a fridge, a pump, an air conditioner, a power tool, a heater, lights) to get the right kind of row with what to read from its nameplate. Group identical loads with the quantity.
- For an appliance, enter its running watts and, if it has a surge, its starting watts. For a motor, enter its output, efficiency and power factor, its locked-rotor data and the starter it uses.
- Check the settings: the design margin, the set’s rated power factor (its kW rating ÷ kVA rating) and the inverter’s or UPS’s power factor. Edit the list of set ratings to the range you can buy, and optionally enter a set to check.
- Read the minimum rating, the worst start and the checks. With a pass, or after unlocking this result, Copy the summary or download it as CSV or a text report.
Examples
10 hp, 88% efficient, power factor 0.85, three-phase
Running 8.47 kW, 9.97 kVA; locked rotor 10 × 6.29 = 62.9 kVA. Minimum set 12.7 kVA with a 20% margin, but it must start 62.9 kVA: with a star–delta starter the start falls to 21.0 kVA
20% margin, set rated at 0.8 power factor
Running 13.8 kW, 15.7 kVA; the pump’s start sets the peak at 42.0 kVA (52.3 kVA if both motors started at once); minimum set 20.7 kVA → 25 kVA from the list
Refrigerator 150 W (1,200 W starting), lights 300 W, TV and router 200 W, 1 hp single-phase pump with 35 A locked-rotor current at 230 V
Running 1.72 kW; peak 8.7 kVA while the pump starts. A portable or inverter generator needs about 2,060 W running and 8,710 W starting
How the size is worked out
- Running load: each load’s kW and its reactive power kW × tan(arccos PF) are added; the running kVA is √(ΣkW² + Σkvar²)
- Worst start: the load whose start adds the most kVA starts last, with everything else running: peak = running kVA + (starting kVA − running kVA) of that load. Starting everything at once adds every surge
- Set rating: the set must carry the running kW with the margin at its rated power factor, and the running kVA with the margin: kVA ≥ max(kW × (1 + margin) ÷ rated PF, kVA × (1 + margin))
- Motor starting: a set large enough for the running load can still stall or drop the voltage too far while a big motor starts. Its data sheet gives the starting kVA it can supply at a given voltage dip; it must reach the peak shown
Set ratings come as kVA and kW: their ratio is the rated power factor, for example 100 kVA and 80 kW at 0.8.
Motor starting kVA: code letters, LRA and starters
A motor started direct on line draws its locked-rotor current until it speeds up. NEMA motors carry a code letter for the locked-rotor kVA per horsepower (NEMA MG 1; the same table is NFPA 70 Table 430.7(B)): for example F 5.0–5.59, G 5.6–6.29, H 6.3–7.09, J 7.1–7.99, K 8.0–8.99 kVA per hp. The calculator uses the upper bound of the letter. Air-conditioner compressor nameplates, and some other motors, give the locked-rotor amps (LRA) instead: starting kVA = √3 × V × LRA ÷ 1000 for three-phase, V × LRA ÷ 1000 for single-phase. IEC motor data sheets give the starting current as a multiple of the rated current, I_A ÷ I_N.
- Star–delta: the windings start in star, so the line current is one third of direct on line
- Autotransformer at tap x: the line current is x² of direct on line (42% at a 65% tap)
- Soft starters and drives limit the current to a setting, entered as a multiple of rated current
Inverters and UPS units
An inverter or UPS has a VA rating and a W rating; their ratio is its power factor (1,000 VA and 800 W is 0.8). It must cover the running VA and the running watts with the margin, and its surge rating, which it can hold only for a short time (see its data sheet), must cover the peak while the largest load starts. Motors with large surges (well pumps, compressors) often need a much larger inverter than their running watts suggest.
Sources
- IEEE Std 446 (the Orange Book): emergency and standby power systems
- NFPA 70, the US National Electrical Code: Table 430.7(B), locked-rotor indicating code letters
- NEMA MG 1, Motors and Generators: locked-rotor code letters
Limitations
- The voltage dip during a motor start depends on the set’s alternator, excitation and governor; only the set’s data sheet (or its maker’s sizing software) gives it.
- Non-linear loads (drives, rectifiers, UPS units, LED drivers) distort the voltage and may need a larger alternator; their makers state how much.
- Single-phase loads on a three-phase set should be shared between the phases; the calculator assumes they are balanced.
- Sets are derated for altitude and high air temperature; apply the maker’s derating to the rating chosen.
- Battery capacity and autonomy of an inverter or UPS are not sized here.
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Frequently asked questions
What do I get without a pass?
Without a pass, Generator & Inverter Size Calculator shows each check’s title, a watermarked starting chart and the names of your first loads (up to 3), with the rating, the working and every figure hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment that never renews, unlocks the full result. The pricing page lists the passes and their prices.
What size generator do I need?
Add up the running watts (or kW) of everything that runs at once, add the extra starting surge of the largest motor, and choose a set whose running rating covers the total with a margin and whose surge or motor-starting rating covers the peak. Enter your loads above to get both figures.
What are starting watts?
Motors draw several times their running current for a moment while they start. Appliances with motors or compressors (refrigerators, pumps, air conditioners, power tools) list a starting or surge figure; heaters, lights and electronics have none.
How do I find a motor’s starting kVA?
From its nameplate: multiply the horsepower by the kVA per hp of its NEMA code letter, or use its locked-rotor amps: √3 × volts × LRA ÷ 1000 for three-phase. A 10 hp motor with code letter G draws up to 62.9 kVA when started direct on line.
Why do generators have both a kVA and a kW rating?
The engine limits the real power (kW) and the alternator the current, which sets the kVA. Their ratio is the rated power factor, for example 80 kW and 100 kVA at 0.8. A load with a lower power factor than the set’s uses up the kVA before the kW.
Should motors start one at a time?
Yes, where you can: with time delays or a start sequence only one surge adds to the running load. In the workshop example above, starting the pump and the compressor together would need 52.3 kVA instead of 42.0 kVA.